Method for monitoring impedance to control power and apparatus utilizing same

ABSTRACT

A method for monitoring the impedance in a circuit coupling a radio frequency electrode to a radio frequency generator to control the power supplied by the generator to the electrode during a treatment procedure. In the method, the impedance in the circuit is monitored over a length of time. An expected impedance at the end of the treatment procedure is calculated from the monitored impedance and compared to a predetermined maximum impedance. The power supplied to the circuit is reduced if the expected impedance is greater than the predetermined maximum impedance. A computer-readable memory and apparatus utilizing the method are provided.

FIELD OF THE INVENTION

This invention pertains generally to methods and apparatus for treatingtissue and, more particularly, to methods and apparatus for treatingtissue utilizing radio frequency energy.

BACKGROUND

Medical devices have been provided for treating tissue of a mammalianbody by the use of radio frequency energy. See, for example, U.S. Pat.Nos. 5,370,675, 5,385,544 and 5,549,644. Radio frequency energy passingfrom an electrode of such a device through the adjoining tissue causesheating of the tissue. In a monopolar device, the radio frequency energypasses from the active electrode to an indifferent electrode typicallyin contact with the exterior of the body of the patient. In order tocause desired tissue ablation and subsequent necrosis, the treatedtissue is heated to a temperature in excess of approximately 47° C.However, if the temperature of the tissue being treated is elevated toohigh, dehydration and later charring of the tissue can occur. Suchdehydration and charring can increase the impedance of the tissue to alevel that prohibits radio frequency from traveling through the tissue.In view of the foregoing, prior systems have monitored the impedance ofthe active electrode circuit and adjusted the radio frequency powersupplied to the electrode in response to such impedance measurements.

It would be desirable to provide a method and apparatus that is able topredict whether the impedance of an active electrode circuit will exceeda predetermined level during a procedure and adjust the power suppliedto such circuit so that such predetermined level of impedance is notreached during the procedure.

SUMMARY OF THE INVENTION

A method is provided for monitoring the impedance in a circuit couplinga radio frequency electrode to a radio frequency generator to controlthe power supplied by the generator to the electrode during a treatmentprocedure. In the method, the impedance in the circuit is monitored overa length of time. An expected impedance at the end of the treatmentprocedure is calculated from the monitored impedance and compared to apredetermined maximum impedance. The power supplied to the circuit isreduced if the expected impedance is greater than the predeterminedmaximum impedance. A computer-readable memory and apparatus utilizingthe method are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of an apparatus and system utilizingthe method for calculating impedance of the present invention.

FIG. 2 is a top elevational view of the apparatus and system of FIG. 1taken along the line 2-2 of FIG. 1.

FIG. 3 is a bottom elevation view of a portion of the apparatus of FIG.1 taken along the line 3-3 of FIG. 1.

FIG. 4 is a cross-sectional view of the apparatus of FIG. 1 taken alongthe line 4-4 of FIG. 1.

FIG. 5 is an enlarged view of the distal extremity of the apparatus ofFIG. 1 wherein the first and second stylets of the apparatus arepartially deployed.

FIG. 6 is an enlarged cross-sectional view of a portion of the apparatusshown in FIG. 4.

FIG. 7 is a circuit diagram of the radio frequency electrodes of theapparatus of FIG. 1 when disposed in tissue being treated.

FIG. 8 is a flow chart of the method for monitoring impedance to controlpower of the present invention.

FIG. 9 is a graph of impedance versus time in a procedure utilizing themethod of FIG. 8 and the apparatus and system of the present invention.

FIG. 10 is a graph of temperature versus time in a procedure utilizingthe method of FIG. 8 and the apparatus and system of the presentinvention.

DESCRIPTION OF THE INVENTION

The method and apparatus of the present invention are for treating amammalian body such as a human patient. Such apparatus is part of asystem 11 and can be in the form of a transurethral needle ablationapparatus or device 12 similar to the apparatus shown in U.S. Pat. No.5,964,756 and in U.S. patent application Ser. No. 09/684,376 filed Oct.5, 2000, the entire content of each of which is incorporated herein bythis reference. Device 12 includes a reusable handle 13 on which thereis mounted a detachable cartridge 14. The needle electrodes of thedevice are supplied with radio frequency energy from a radio frequencygenerator and controller 16, which can be similar to the typecommercially available from Medtronic, Inc. of Minneapolis, Minn. Thedevice 12 is further supplied with a conductive liquid such as a salinesolution provided from one or more reservoirs and preferably from asaline supply 17 (see FIG. 2). Controller 16 is preferably coupled tothe saline supply 17 to control the output thereof. The method andapparatus of the present invention can be utilized to monitor theimpedance in the electrode circuits so as to control the amount of radiofrequency energy supplied to the needle electrodes of the apparatus.

Apparatus 12 is similar in construction to the apparatus disclosed inU.S. Pat. No. 5,964,756. Using that same construction, handle 13 iscomprised of a housing 21 which is ergonomically shaped so as to beadapted to fit in a human hand. Specifically, the handle 13 is in theform of a pistol grip which has a main body portion 22 that is providedwith a forward indentation 23 adapted to receive the index finger of thehuman hand grasping the handle 13 and a larger rearwardly facingindentation 24 adapted to receive the thumb of the same human hand.Housing 21 is made from metal or any other suitable material.

Cartridge 14 consists of a cover 31 that is generally U-shaped in crosssection and is formed of a suitable material such as plastic. The cover31 is provided with proximal and distal extremities 31 a and 31 b and isformed by a curved top wall 32 and depending adjoining spaced-apartparallel side walls 33. A release button 34 is provided on each of theopposite sides of the housing 21 for releasing the removable cartridge14 from the handle 13.

An elongate tubular member or probe 41 preferably in the form of a rigidtorque tube made from any suitable material such as stainless steel isprovided and includes proximal and distal extremities 41 a and 41 b.Probe 41 has its proximal extremity mounted to the distal extremity 31 bof cover 31. The tubular torque member 41 has a suitable diameter as forexample 18.5 French and is provided with a passage 42 circular in crosssection extending therethrough (see FIG. 3). The outer surface of theprobe 41 is provided with spaced-apart markings 43 which are spacedapart by one centimeter increments to aid the physician in insertion ofthe probe 41 into the urethra.

A bullet-shaped tip or distal guide housing 46 formed of a suitableplastic transparent to light is secured to the distal extremity of thetorque tube or probe 41 in the manner described in U.S. Pat. No.5,964,756 (see FIGS. 1 and 3). As shown in FIG. 1, the distal tip 46 hasan upturned rounded portion 46 a. The elongate probe 41 and the tip 46preferably have a combined length of approximately 9.5 inches. A pair ofcircumferentially spaced-apart holes 47 and 48 are provided on theunderside of the bullet-shaped tip 46 opposite the upturned portion 46a. The first and second holes 47 and 48 are spaced apart from each otherby a suitable distance as for example one centimeter, which dimension isdetermined by the diameter of the torque tube 46 (see FIG. 3). First andsecond angled guide tubes 51 and 52 which are aligned with therespective first and second holes 47 and 48 have L-shaped 90° bendstherein that are molded into the transparent bullet-shaped tip 46. Such90° bends provided in the first and second angled guide tubes providetransitions from movement through the tubes along a longitudinal axis tomovement along a transverse axis extending at 90° with respect to thelongitudinal axis.

The first and second angled guide tubes 51 and 52 adjoin straight guidetubes 56 and 57, respectively, which extend through the passage 42provided in the torque tube or elongate probe 41 (see FIGS. 3 and 4).Each of the straight guide tubes 56 and 57 has a proximal extremityattached to cover 31 and a distal extremity attached to the distal tip46. As shown particularly in FIG. 4, the straight guide tubes 56 and 57are supported in predetermined spaced-apart positions in the passage 42by an insert 58 formed of plastic that is disposed in the torque tube 41and has spaced-apart recesses 59 formed in the outer periphery of theinsert 58. The straight guide tubes 56 and 57 are made from plastic orany other suitable material.

A pair of first and second elongate members or stylets 66 and 67 areslidably mounted in the first and second straight guide tubes 56 and 57within probe 41 (see FIGS. 4-6). Each of the elongate stylets has aproximal extremity, not shown, disposed in cover 31 and a distalextremity 68 disposed in the distal extremity of probe 41 and tip 46.First stylet 66 is preferably formed from a needle electrode 71 and alayer of insulating material disposed around the needle electrode butexposing a distal portion of the needle electrode. The layer ofinsulating material is preferably a sleeve 72 slidably mounted on theneedle electrode 71. Second stylet 67 is similar in construction to thefirst stylet 66 and includes a needle electrode 73 and a sleeve 74slidably mounted on the needle electrode 73. The needle electrodes 71and 73 are preferably formed of a hollow superelastic nickel-titaniummaterial having an outside diameter of 0.018 inch and an inside diameterof 0.012 inch and a wall thickness of 0.003 inch. The sleeves 72 and 74are preferably made from plastic or any other suitable insulatingmaterial and extend through the guide tubes 51, 52, 56 and 57 so thatthe entire lengths of the needle electrodes 71 and 73 extending throughthe passage 42 are insulated from each other and from the torque tube41. The sheaths or sleeves 72 and 74 additionally provide stiffness tothe needle electrodes during penetration of the urethral or otherpassage wall into which tip 46 is introduced. The insulating sheaths aresized in length so that when the needle electrodes are retracted withinthe bullet-shaped tip 46, they are substantially covered with theinsulation. When the needle electrodes are deployed, the sheaths 72 and74 continue to cover the needle electrodes, but permit the distalportion of the needle electrodes to be exposed in the targeted tissue.The stylets 66 and 67 have an included angle of approximately 40°.

A suitable temperature sensor is optionally carried by each of the firstand second stylets 66 and 67. The distal extremity of each of the needleelectrodes is provided with a sharpened tip and has a thermocouple 76 orother suitable temperature sensor mounted within the sharpened tip (seeFIG. 5). Each thermocouple is provided with a pair of wires 77 and 78which extend proximally from the sharpened tip through a longitudinallumen 81 provided in the hollow needle electrode 71 or 73 (see FIGS. 4and 6). A separate insulating sleeve 82 is provided in each electrodelumen 81 to provide additional insulation isolating the thermocouplewires from the metal needle electrode. In order to strengthen the needleelectrodes 71 and 73 and to inhibit wall collapse and kinking duringbending, a nickel-titanium rod 83 is disposed within each internalsleeve 82 alongside the thermocouple wires 77 and 78. Strengthening rod83 has an external diameter of 0.006 inch and each of the thermocouplewires 77 and 78 has an outside diameter of 0.005 inch. The rod 83 andthe thermocouple wires 77 and 78 are cemented in place by a suitablepolyurethane adhesive (not shown).

Handle 13 and cartridge 14 are provided with internal mechanisms muchthe same as described in U.S. Pat. No. 5,954,756, wherein the operationof such mechanisms are described in detail. In general, such mechanismsare adapted to be operated by a needle and sheath deployment andretraction trigger 91 that is adapted to be engaged by the forefinger ofthe hand holding the body portion of the housing 21 (see FIG. 1). Thetrigger 91 is adapted to be moved from a “load” position indicated bythe arrow 92 through a plurality of deployed positions indicated byindicia 93 ranging from 12 to 22 millimeters provided on opposite sidesof the housing 21. In this regard, actuation of the trigger 91 initiallycauses the first and second stylets 66 and 67 to slidably deploy fromrespective guide tubes 51 and 56 and 52 and 57 so as to extend sidewisein unison from the distal tip. Further downward movement of the trigger91 causes the insulating sleeves 72 and 74 to retract a predeterminedamount relative to the respective needle electrodes 71 and 73. Thelength of the resulting tissue penetration of stylets 66 and 67 isdetermined by the position of an interconnected pair of knobs 96, whichset stops for limiting movement of the trigger 91 so that overtravelbeyond the setting provided by the knobs 96 cannot occur. Theinterconnected knobs 96 are provided on opposite sides of the housing 21adjacent the lower extremity of the body 21 and have pointers 97 movableover indicia 98 ranging from 12 to 22 millimeters in the same incrementsas the indica 93. The indicia 98 indicate the length of penetration ofthe needle electrodes 71 and 73, for example through the urethral walland into the prostatic tissue of the prostate. Sleeves or sheaths 72 and74 are retracted a predetermined amount as for example six millimetersrelative to the needle electrodes so that there is exposed approximatelysix millimeters of the needle electrodes in the targeted tissue with theinsulating sheaths still extending through the urethral or other passagewall so as to protect such wall during RF ablation of the targetedtissue.

Generator and controller 16 is electrically coupled to the first andsecond stylets 66 and 67, and specifically to the first and secondneedle electrodes 71 and 73. In this regard, an electrical connector 101is provided on cover 31 for permitting electrical communication betweenthe generator 16 and the proximal extremity of the needle electrodes.Controller 16 is electrically coupled to connector 101 by means of acable 102 or other suitable lead. The generator 16 is provided with twochannels of radio frequency energy, making it possible to deliverdifferent amounts of power to two or more different needle electrodeswhich are typically operated in a monopolar fashion utilizing a returnor dispersive electrode 103 which can be adhered to exterior of the bodyof the patient, for example the small of the back of the patient. Theproximal ends of first and second thermocouple wires 77 and 78 are alsoelectrically coupled to connector 101 for permitting controller 16 tomonitor temperatures sensed thereby.

An optional temperature sensor such as a thermocouple 106 is preferablyencapsulated in the bullet-shaped tip 46 and, as shown in FIG. 3, isdisposed in the vicinity of stylet openings 47 and 48 provided in thetip. Thermocouple 106, which permits the sensing of urethral walltemperatures, is connected to wires 107 and 108 extending through thepassage 42 and is supported in a recess 109 in the insert 58 (see FIG.4). The wires 107 and 108 are electrically connected within cover 31 toconnector 101 for permitting the monitoring of the readings obtainedthereby by generator and controller 16. The thermocouple 106 is used toensure that the highest temperature reached in the urethra does notexceed approximately 47° C. Such hottest location is typically foundbetween the needle pairs 71 and 73 and it is for this reason that thethermocouple 106 is so located.

The cover 31 and the torque tube 41 are preferably sized to receive anoptional telescope or scope 116 of a conventional type which includes atubular member 117 having a rod lens 118 and fiber optics (not shown)surrounding the rod lens (see FIGS. 1 and 2). The scope 116 is movablethrough the cover 31 and a recess 119 provided in the insert 58 disposedin the passage 72 of the tube 41 and thence into a bore 121 provided inthe bullet-shaped tip 46 (see FIG. 3). The bore 121 is in alignment withthe recess 119 provided in the torque tube 41. When the distal extremityof the tubular member 117 is positioned within the bore 121, it ispossible to view the surrounding region through the transparent tip 46because the tip 46 has an index of refraction which is similar to thesurrounding liquid, such as saline solution, within the urethra or otherbody passage into which probe 41 has been placed. A fitting 122 isprovided on the proximal extremity of the tubular member 117 andincludes an eyepiece 126 and a connector 127 for making connection to afiber optic light source (not shown).

In order to permit movement of the scope 116 into position so that thephysician can also observe independently deployment of the first andsecond needle electrodes 71 and 73, optional means is preferablyprovided for causing longitudinal movement of the scope 116 relative tothe torque tube 41 (see FIGS. 1 and 2). To this end telescope movingmeans 131, described in detail in copending patent application Ser. No.09/684,376 filed Oct. 5, 2000 is provided in the proximal extremity 31 aof cover 31. In general, the telescope moving means 131 includes atelescope positioning knob 132 extending from one of the side walls 33of cover 31 and a scope locking lever 133. Release button 34, and theinternal mechanisms and operation thereof, are also described incopending patent application Ser. No. 09/684,376 filed Oct. 5, 2000.

Each of the first and second stylets 66 and 67 optionally has a lumenextending from the proximal extremity to the distal extremity of thestylet for permitting a conductive or other fluid to be introduced byapparatus 12 into the tissue being treated. The lumen can be provided inany portion of the stylet and can be in the form of a lumen extendingthrough the needle electrode or through the insulating sleeve. In onepreferred embodiment, and as shown in the drawings, each of theinsulating sleeves 72 and 74 is provided with a lumen 136 extendinglongitudinally therethrough. As shown in FIG. 6, the lumen can be anannular lumen 136 extending around the respective needle electrode andpermitted by sizing the internal diameter of the insulating sleevelarger than the external diameter of the needle electrode.Alternatively, or in addition, the lumen can be in the form of one ormore lumens 136′, one of which is shown in dashed lines in FIG. 6, whichare offset from the central lumen of the sleeve 72. Where more than onelumen 136′ is provided, such lumens can be spaced circumferentially orotherwise about the insulating sleeve.

The lumen 136 is accessible from the proximal extremity of therespective stylet and a reservoir 17 of a suitable conductive liquidsuch as saline is coupled to the proximal extremity of each stylet forsupplying such liquid to the tissue targeted by apparatus 12 (see FIG.2). One or more suitable fluid connectors 138 are provided on apparatus12 for permitting fluid communication between reservoir or saline supply17 and sleeve lumens 136. In the illustrated embodiment of theinvention, first and second fluid connectors in the form of first andsecond stopcocks 138 extend from the opposite side walls 33 of the cover31 and connect to saline supply 17 by means of suitable lines or tubing,shown in dashed lines in FIG. 2.

A circuit diagram of system 11 when first and second stylets 66 and 67are exposed in tissue targeted for treatment is shown in FIG. 7, wheretargeted tissue 141 of a mammalian body 142 are also depicted. Ingeneral, first circuit 146 therein depicts the electrical circuitcreated by controller 16, cable 102, apparatus 12 and body 142 of thepatient pertaining to first stylet 66 and extending from the voltagesource V_(F) ⁺ for such stylet 66 within controller 16 to the return orindifferent electrode 103 preferably adhered to the back side of thepatient. Similarly, second circuit 147 depicts the electrical circuitcreated by controller 16, cable 102, apparatus 12 and body 142pertaining to second stylet 67 and extending from the voltage sourceV_(S) ⁺ to the disbursive electrode 103. Second circuit 147 is distinctfrom first circuit 146 in that there is no physical connection betweensuch circuits. Nodes 148 and 149 respectively refer to the locationswhere first and second circuits 146 and 147 connect through cable 102 tocontroller 16. Nodes 151 and 152 respectively refer to the exposedportions of first and second needle electrodes 71 and 73 within thetargeted tissue 141.

The impedance for first circuit 146 within controller 16 is depicted byreference R_(CF) in FIG. 7. The impedance between node 148 and node 151is depicted by reference R_(DF) and the impedance of body 142, that isbetween node 151 and return electrode 103, is depicted by referenceR_(BF). Similarly, second circuit 147 has respective impedancecomponents R_(CS), R_(DS), and R_(BS).

Controller 16 includes a central processing unit or central processor156 and a computer memory 157 electrically coupled to such centralprocessor or CPU 156. Computer-readable memory 157 includes a programfor performing the method of the present invention, which is set forthin the flow chart of FIG. 8 and described below.

In one method for treating tissue of the present invention, system 11can be used to treat benign prostatic hyperplasia in a human maleprostate. In such a procedure, the targeted tissue 141 is the prostatictissue of a prostate. A suitable procedure for treating a prostate of ahuman male is described in detail in U.S. Pat. Nos. 5,549,644 and5,964,756, the entire contents of which are incorporated herein by thisreference. In general, the distal extremity of torque tube 41 ofapparatus 12 is introduced through the penis into the urethra untildistal tip 46 is in the vicinity of the prostate. The operatingphysician then pulls down on trigger 91 to cause the first and secondstylets 66 and 67 to deploy from distal tip 46. The sharpened tips offirst and second needle electrodes 71 and 73 penetrate the urethral wallto permit the stylets to extend into the prostatic tissue 141 to betreated. As discussed above, further downward movement of trigger 91causes first and second sleeves 72 and 74 to retract relative to theelectrodes. The sleeves, however, extend through the urethral wall so asto protect the wall from radio frequency energy supplied to the needleelectrodes 71 and 73.

If the operating physician desires to create a wet electrode within theprostatic tissue 141, a procedure such as described in copending U.S.patent application Ser. No. ______ filed Jul. 22, 2002 [Attorney docketno. A-70947], the entire content of which is incorporated herein by thisreference, can be utilized. In general, a conductive liquid provided bysupply 17 is introduced through first and second stylets 66 and 67 intothe prostatic tissue 141 to form such a wet electrode about each of thefirst and second stylets 66 and 67. The exterior surface of each suchwet electrode serves as an outer electrode surface from which radiofrequency energy is delivered.

Radio frequency energy is supplied by radio frequency generator andcontroller 16 to first and second needle electrodes 71 and 73 to createlesions in the prostatic tissue 141 by ablating the tissue. The methodof the invention set forth in FIG. 8 applies to both electrodes 71 and73 but, for simplicity, is described with respect to only one of theelectrodes. Step 161 corresponds to the initial supply of radiofrequency energy to such electrode 71 or 73. The amount of power sosupplied to the electrode can be predetermined or estimated to raise thetemperature of the prostatic tissue above 47° C. and preferably toapproximately 110° C., as shown in FIG. 10. Although the power can beramped over time to such predetermined power level, in one preferredembodiment the power is raised from zero to such predetermined powerlevel in a stepwise fashion. A suitable initial power applied to each ofthe electrodes 71 and 73 in such stepwise fashion has been found to be15 watts.

In step 162 set forth in FIG. 8. controller 16 and the operator thereofwait for a length of time for certain parameters of the procedure, suchas the temperature of the targeted tissue 141, to stabilize. As shown inthe exemplary data contained in FIG. 10, the temperature of theprostatic tissue rises to the desired temperature of 110° C. inapproximately 30 or 40 seconds. The length of such wait is dependent, atleast in part, on the amount of the electrode 71 or 73 that is disposedin the targeted tissue. Such amount of exposed surface, dependent on thediameter and length of the exposed portion of the electrode, is relevantin determining the current density at the surface of the electrode. Thesmaller the current density, the longer it takes to heat the tissue 141to the targeted tissue for a given power. The temperature sensed by thethermocouple 76 in each of the first and second needle electrodes 71 and73 can be displayed on a meter or other visual display on controller 16.

The impedance of the first and second circuits 146 and 147 is similarlymonitored during the wait of step 162. In one preferred method of theinvention, particularly where cross talk exists between adjacentelectrodes disposed in the targeted tissue, the impedance is monitoredby the method set forth in copending U.S. patent application Ser. No.______ filed Jul. 22, 2002 [Attorney Docket No. A-71481]. As shown inFIG. 9, the impedance in the circuit 146 or 147 decreases during theinitial portion of the procedure. This is due to the breaking down orbursting of some prostatic cells. The resulting release of fluid fromthese cells increases the hydration of the targeted tissue 141.Accordingly, the decrease in impedance at the start of the procedureresults from a decrease in R_(BF) or R_(BS), that is the portion of theimpedance between the respective needle electrode 71 or 73 and theindifferent electrode 103. Thereafter, it has been found that in someprocedures the impedance of the targeted tissue 141 increases at asubstantially constant rate throughout the procedure when thetemperature of such tissue is maintained at a constant temperature suchas 110° C. or below. Controller 16 can include a meter or other visualdisplay thereon for showing the measured impedance in each of first andsecond circuits 146 and 147.

In decision step 163 shown in FIG. 8, controller 16 determines whethercertain parameters of the procedure have stabilized after the wait ofstep 162. For example, if fluctuations in the temperature of theprostatic tissue or in the impedance of the respective circuit existafter the wait of step 162, controller 16 returns to step 162 for anadditional wait.

In one preferred method of the invention, the wait of step 162 lastsapproximately one second and the controller travels through the loop ofsteps 162 and 163 until the temperature of the targeted tissue and theimpedance of the electrode circuit 146 or 147 stabilize. In anotherpreferred method of the invention, step 163 is eliminated and insteadthe controller waits for a predetermined period of time ranging from 20to 60 seconds and preferably from 40 to 60 seconds in step 162 beforemoving directly to step 166. When step 163 is eliminated, thepredetermined period of time for step 162 is set so as to be sufficientfor such procedure parameters to stabilize.

Throughout the duration of the procedure, the temperature of thetargeted tissue 141 is preferably maintained constant so as tofacilitate an accurate prediction of the impedance expected to exist inthe circuit 146 or 147 at the end of the procedure. It is preferred thatsuch constant tissue temperature be at least 110° C. and preferablyapproximately 110° C. when the targeted tissue is prostatic tissue sinceit has been found that an undesirable increase in the impedance of theprostatic tissue occurs when the temperature approaches and exceedsapproximately 125° C. When a constant tissue temperature ofapproximately 110° C. is maintained, it has been additionally found thatan acceptable impedance level exists in many prostates to permit arelatively high current density on the electrode surface and thus arelatively rapid introduction of energy into the prostate.

Changes in the power supplied to the electrode are typically necessaryto maintain the prostatic temperature at a relatively constanttemperature throughout the remainder of the procedure. In this regard,controller 16 can include a program within memory 157 or be otherwiseprogramed to automatically reduce the radio frequency energy supplied toa needle electrode 71 or 73 in response to undesirable changes in thetemperature adjacent such needle electrode. In addition, the operatorcan manually adjust the amount of radio frequency energy being suppliedto a needle electrode in response to such temperature readings.

The program within memory 157 next directs controller 16 to move to step166 in FIG. 8 to monitor the impedance, and specifically the change ofimpedance and more specifically the rate of change of impedance, in thecircuit over a length of time. In FIG. 9, for example, step 166commences at approximately 52 seconds into the procedure and the lengthof time, depicted by L₁, is approximately 20 seconds. The length of timefor step 166 is of any suitable length and can range from ten to 120seconds. As discussed above, the temperature of the targeted tissue inthe vicinity of the electrode being monitored is preferably maintainedconstant during the measuring step 166.

In step 167 of the invention, the program within memory 157 directscontroller 16 to calculate an expected impedance at the end of thetreatment procedure. In the procedure set forth in FIG. 8, controllerutilizes the change of impedance over time determined in step 166 tocalculate the expected impedance at the end of the procedure. In thepreferable situation where the rate of change of impedance over time isrelatively constant, the expected impedance at the end of the procedurecan be determined by extrapolation. In FIGS. 9 and 10, for example, theprocedure is shown as being approximately three minutes, 15 seconds induration. The extrapolation in step 166 is shown by line 168, whichpredicts an impedance of greater than 300 ohms at the end of thetreatment procedure. Controller can be additionally programmed tocalculate the expected impedance at the end of the treatment period byvarious other known algorithms such as least squares or polynomials.

In decision step 171, the expected impedance calculated in step 167 iscompared to a predetermined maximum impedance. Although suchpredetermined maximum amount can be of any suitable value, for examplebetween 200 and 300 ohms, it is preferably chosen to be the level atwhich undesirable dehydration of the targeted tissue 141 occurs. Asdiscussed above, such dehydration inhibits if not precludes the passageof further radio frequency energy through the targeted tissue and canthus undesirably limit the size of the resulting lesion. In onepreferred procedure, the predetermined maximum impedance is 250 ohms.

If controller 16 predicts in step 171 that the expected or estimatedimpedance in the respective circuit 146 and 147 will equal or exceed thepredetermined maximum impedance at or before the end of the procedure,then the method of the present invention continues to step 172 of FIG. 8wherein controller 16 directs that the temperature of the targetedtissue 141 be reduced. In one preferred method of the invention, thetemperature is reduced 2° C. for each 20 ohms that the expectedimpedance exceeds 250 ohms until a minimum temperature of 98° C. isachieved. Such reduction in temperature, shown in FIG. 10 where thesensed temperature of the tissue 141 is reduced approximately 4° C. atapproximately 1 minute, 12 seconds into the procedure, is accomplishedby a reduction in power supplied to the needle electrode. Thereafter,the controller 16 is directed to return to step 162.

If the method of the invention predicts that the expected impedance willnot equal or exceed the predetermined maximum impedance after theinitial 20 second observation window of step 166, then step 171 directscontroller 16 to return to step 166 and continue in the loop of steps166, 167 and 171 so as to expand the observation window during which thechange of impedance is monitored and the expected impedance at the endof the procedure calculated. Longer observation windows provide moredata for the calculation of the expected impedance at the end of theprocedure and thus contribute to the reliability of the predictedexpected impedance. If any time during such loop the expected impedanceis determined in step 171 to exceed the predetermined maximum impedance,then controller 16 moves to step 172. Alternatively, if the expectedimpedance from step 167 continues to remain below the predeterminedmaximum impedance, then the procedure continues to completion withoutfurther reductions to the temperature of the targeted tissue,accomplished by reductions in the amount of radio frequency powersupplied to the needle electrode.

If the controller is directed by step 172 to return to step 162, thecontroller 16 waits for an interval of time for the procedure parametersto stabilize. In one preferred method of the invention, the controlleris directed to wait for five seconds for the temperature of the targetedtissue and the impedance in the electrode circuit to stabilize from thereduction in power directed by step 172.

After the procedure parameters have stabilized, including instanceswhere controller 16 returns to step 162 for an additional wait toaccomplish such stabilization, the controller returns to step 166 toremeasure the rate of change of impedance in the circuit over anadditional length of time, identified as L₂ in FIG. 9. As discussedabove with respect to L₁, the length of time L₂ can be of any suitablelength of time and, for example, can be a length of time equal to ordifferent than length of time L₁. In the example of FIG. 9, theremeasuring of the rate of change of impedance commences at one minute,45 seconds into the procedure and lasts approximately 20 seconds untiltwo minutes, 5 seconds into the procedure. As shown in FIG. 10, thetemperature of the targeted tissue 141 remains constant during suchmeasurement period L₂.

The program within memory 157 next directs controller 16 to step 167where a new expected impedance at the end of the treatment procedure iscalculated from the remeasured rate of change of impedance previouslydetermined in step 166. In FIG. 9, for example, the controller usesextrapolation line 173, whose slope is equal to the remeasured rate ofchange of impedance, to predict that the new expected impedance will beapproximately 250 ohms at the end of the procedure. Thereafter, thecontroller moves to decision step 171 where the new expected impedanceis compared to the predetermined maximum impedance discussed andutilized above. Since the new expected impedance is equal to thepredetermined maximum impedance value of 250 ohms, controller 16 movesto step 172 wherein the targeted temperature is decreased resulting in afurther reduction in the power to the needle electrode. Such temperaturereduction appears in FIG. 10 at two minutes, five seconds into theprocedure.

Steps 162, 163, 166 and 167 are further repeated by controller 16 beforethe controller again returns to step 171 to determine whether a furtherrecalculated expected impedance at the end of the procedure will begreater than or equal to the predetermined maximum. In the example ofFIGS. 9 and 10, controller 16 measures the rate of change of impedanceover the length of time L₃, commencing at approximately two minutes, 38seconds into the procedure. In step 166 the controller thereaftercalculates a further new expected impedance at the end of the procedurethrough third extrapolation line 176 shown in FIG. 9. As can be seenfrom FIG. 9, the further new expected impedance ohms at the end of theprocedure is less than 250 ohms and, thus, controller 16 decides in step171 to continue in the loop of steps 166, 167 and 171 in the manner setforth above.

The procedure and apparatus of the invention desirably maintain tissueimpedance at acceptable levels throughout the procedure. In doing so,hydration of the tissue is not reduced so as to inhibit the passage ofradio frequency energy from a radio frequency electrode to the returnelectrode. The inhibition of dehydration permits relatively hightemperatures to be maintained throughout the procedure. Such hightemperatures contribute to reducing the time of the procedure, which isdesirable to the patient.

Although the procedure and apparatus of the invention have beendescribed in the context of procedures where the impedance increaseslinearly during the procedure, the procedure and apparatus of theinvention can also be utilized in any procedure where the impedance canbe predicted at some future time in the procedure. In this regard, step166 can be eliminated if the expected impedance at the end of theprocedure for the circuit being monitored can be calculated in anotherfashion, for example without the need for determining the rate of changeor other change of impedance over time.

The foregoing procedure of the invention has been described with the useof first and second stylets 66 and 67, however it should be appreciatedthat any plurality of stylets can be utilized. Further, it should beappreciated that the apparatus and system of the present invention canbe of any suitable type having at least first and second activeelectrodes. The method can be utilized in any such apparatus and systemwhere the impedance of at least one of the first and second activeelectrode circuits is monitored and is preferably suited for anapparatus and system utilizing radio frequency energy. Although themethod and apparatus of the invention have been described in connectionwith the treatment of the prostate, such method and apparatus can beused in any tissue of the body.

From the foregoing, it can be seen that a new method and apparatus havebeen provided for monitoring the impedance in a circuit coupling a radiofrequency electrode to a radio frequency generator to control the powersupplied to the electrode. The method and apparatus permit the impedanceof the circuit to be predicted so as to determine whether the impedanceis expected to remain within acceptable levels for the remainingduration of the procedure. If unacceptably high impedance levels arepredicted, power to the radio frequency electrode is reduced.

1. A method for monitoring impedance in a circuit coupling a radiofrequency electrode to a radio frequency generator to control powersupplied by the generator to the electrode during a treatment procedurecomprising the steps of monitoring the impedance in the circuit over alength of time, calculating an expected impedance at the end of thetreatment procedure from the monitored impedance, comparing the expectedimpedance to a predetermined maximum impedance and reducing the powersupplied to the circuit if the expected impedance is greater than thepredetermined maximum impedance.
 2. The method of claim 1 wherein themonitoring step includes the step of measuring the change of impedanceover the length of time.
 3. The method of claim 2 wherein the measuringstep includes the step of measuring the rate of change of impedance overthe length of time.
 4. The method of claim 1 further comprising the stepof maintaining the temperature of the tissue constant during themonitoring step.
 5. The method of claim 1 further comprising the step ofwaiting a duration of time for procedure parameters to stabilize beforecommencing the monitoring step.
 6. The method of claim 1 wherein thepredetermined maximum impedance is between 200 and 300 ohms.
 7. Themethod of claim 6 wherein the predetermined maximum impedance is 250ohms.
 8. The method of claim 1 further comprising the steps ofremonitoring the impedance in the circuit over an additional length oftime, calculating a new expected impedance at the end of the treatmentprocedure from the remonitored impedance, comparing the new expectedimpedance to the predetermined maximum impedance and reducing the powersupplied to the circuit if the new expected impedance is greater thanthe predetermined maximum impedance.
 9. The method of claim 1 whereinthe radio frequency electrode is a needle electrode.
 10. The method ofclaim 9 wherein the needle electrode is slidably carried by an elongateprobe member introduceable into the urethra of a human male fortreatment of the tissue of the prostate.
 11. The method of claim 9wherein a temperature sensor is carried by the needle electrode.
 12. Themethod of claim 1 further comprising the step of performing a medicalprocedure on the tissue.
 13. A computer-readable memory for use with aradio frequency controller and a circuit to couple a radio frequencyelectrode to the controller, the memory containing a computer programfor causing the controller to monitor impedance in the circuit so as tocontrol power supplied by the generator to the electrode during atreatment procedure by measuring the change of impedance in the circuitover a length of time, calculating an expected impedance at the end ofthe treatment procedure from the change of impedance, comparing theexpected impedance to a predetermined maximum impedance and reducing thepower supplied to the circuit if the expected impedance is greater thanthe predetermined maximum impedance
 14. The computer-readable memory ofclaim 13 wherein the computer program further causes the controller toremeasure the change of impedance in the circuit over an additionallength of time, calculate a new expected impedance at the end of thetreatment procedure from the remeasured change of impedance, compare thenew expected impedance to the predetermined maximum impedance and reducethe power supplied to the circuit if the new expected impedance isgreater than the predetermined maximum impedance.
 15. A radio frequencycontroller for use with a circuit to couple a radio frequency electrodeto the controller comprising a computer-readable memory containing acomputer program for causing the controller to monitor impedance in thecircuit so as to control power supplied by the generator to theelectrode by measuring the rate of change of impedance in the circuitover a length of time, calculating an expected impedance at the end ofthe treatment procedure from the rate of change of impedance, comparingthe expected impedance to a predetermined maximum impedance and reducingthe power supplied to the circuit if the expected impedance is greaterthan the predetermined maximum impedance, and a central processing unitcoupled to the memory for executing the program in the memory.
 16. Theradio frequency controller of claim 15 wherein the program in the memoryfurther causes the controller to maintain the temperature of the tissueconstant while measuring the rate of change of impedance in the circuitover the length of time.